Does Foreach Automatically Call Dispose

Does foreach automatically call Dispose?

Yes, foreach will call Dispose() on the enumerator if it implements IDisposable.

Enumerator disposal when not using using, foreach or manually calling Dispose()

When enumerating over this iterator, if the enumerator's Dispose() is not explicitly called, and not used within a using statement, would the underlying iterator remain in an open state?

Let me re-phrase that question into a form that is easier to answer.

When using foreach to enumerate via an iterator block that contains a using statement, are the resources disposed of when control leaves the loop?

Yes.

What mechanisms ensure this?

These three:

• A using statement is just a convenient way to write a try-finally where the finally disposes of the resource.

• The foreach loop is also a convenient syntax for try-finally, and again, the finally calls Dispose on the enumerator when control leaves the loop.

• The enumerator produced by an iterator block implements IDisposable. Calling Dispose() on it ensures that all the finally blocks in the iterator block are executed, including finally blocks that come from using statements.

If I avoid the foreach loop, call GetEnumerator myself, and don't call Dispose on the enumerator, do I have a guarantee that the finally blocks of the enumerator will run?

Nope. Always dispose your enumerators. They implement IDisposable for a reason.

Is that now clear?

If this subject interests you then you should read my long series on design characteristics of iterator blocks in C#.

http://blogs.msdn.com/b/ericlippert/archive/tags/iterators/

why these objects are not getting disposed

foreach doesn't call the Dispose method, only using does. using directive is just a sugar for:

try {
    // Initialize
}
finally {
    // Dispose
}

And yes, you have to write Dispose call by youself, like this:

foreach (var e in GetEmployees())
{
    Console.WriteLine(e.Name);
    e.Dispose();
}

or

foreach (var e in GetEmployees())
{
    using (e)
    {
        Console.WriteLine(e.Name);
    }
}

Consider Dispose Pattern from MSDN for better understanding way the Disposing works in .NET:

Simple use-case:

public class DisposableResourceHolder : IDisposable {

    private SafeHandle resource; // handle to a resource

    public DisposableResourceHolder(){
        this.resource = ... // allocates the resource
    }

    public void Dispose(){
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing){
        if (disposing){
            if (resource!= null) resource.Dispose();
        }
    }
}

Complex use-case with finalizible types:

public class ComplexResourceHolder : IDisposable {

    private IntPtr buffer; // unmanaged memory buffer
    private SafeHandle resource; // disposable handle to a resource

    public ComplexResourceHolder(){
        this.buffer = ... // allocates memory
        this.resource = ... // allocates the resource
    }

    protected virtual void Dispose(bool disposing){
            ReleaseBuffer(buffer); // release unmanaged memory
        if (disposing){ // release other disposable objects
            if (resource!= null) resource.Dispose();
        }
    }

    ~ ComplexResourceHolder(){
        Dispose(false);
    }

    public void Dispose(){
        Dispose(true);
        GC.SuppressFinalize(this);
    }
}

Update: As in comments noted, I think that you are mixing up the Garbage Collection and Disposing. Disposing is used for freeing up the unmanaged resources outside the .NET Framework in your application. Garbage Collection is done automatically and you should not force it until you have completely understanding why do you need it.

Proper way to Dispose of IDisposable COM Object Wrappers After foreach Loop

foreach doesn't dispose of all of the objects in the sequence being iterated. It only disposes of the IEnumerator object that provides you with the objects being iterated over.

Do LINQ IEnumerable extensions call Dispose on their IEnumerable?

Yes, you can rely on the call of Dispose() on your iterators from inside the methods of LINQ.

In the reference code published by Microsoft, the use of iterators falls in three categories:

• Iterators are used from within a foreach loop, which ensures a call of Dispose(), or
• Iterators are used in conjunction with a while loop; these iterators are disposed explicitly
• Iterators are obtained inside a using block, which ensures automatic disposal.

In all these cases the library ensures that iterator's Dispose() method is called upon completion.

When is Dispose() of a class implementing IEnumeratorT called automatically by Linq?

None of your code creates any enumerators that you are responsible for disposing of. LINQ operators like Count are designed so that they create and dispose the enumerators internally, so you don't have to worry about it at all.

You can see the reference source for Count here, and you'll find the snippet:

int count = 0;
using (IEnumerator<TSource> e = source.GetEnumerator())
{
    checked
    {
        while (e.MoveNext())
        {
            count++;
        }
    }
}

return count;

The using statement already disposes the enumerator for you - Dispose is called, before Count even returns. There's nothing for you to do.

Of course, if you are consuming stringsFound by getting its enumerator (though it's very rare that you'd need to do it this way), you do need to remember to dispose it.

var stringsFoundEnumerator = stringsFound.GetEnumerator();
//  ^^^^^^ remember to dispose this!

Side note: foreach loops translates to while loops wrapped by a using statement, so you don't need to worry about disposing the enumerator if you are iterating over stringsFound with a foreach loop either. :D

You should not call Dispose more than one time on an object (CA2202)

This code analysis warning is total baloney. The contract for IDisposable requires that extra calls to Dispose are accepted and do nothing (in particular, they should not throw ObjectDisposedException or any other exception).

If an object's Dispose method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its Dispose method is called multiple times. Instance methods other than Dispose can throw an ObjectDisposedException when resources are already disposed.

Source: IDisposable.Dispose documentation on MSDN

Unfortunately some framework code was written without reading the contract, and forbids calling Dispose more than once. Those objects you should be careful not to double dispose. But the universal contract is still that for an arbitrary IDisposable, calling Dispose multiple times is permitted.

Combining foreach and using

foreach (ManagementObject result in results)
using(result)
{
    //code here
}

It's not normally good practice to assign the variable outside the using block because the resource would be disposed but could stay in scope. It would, however, result in clearer code here because you can nested the using statement against the foreach.

EDIT:
As pointed out in another answer, ManagementObjectCollection also implements IDisposable so I have added that into a using block.

No need to place ManagementObjectCollection in a using statement. the foreach will call Dispose() on the enumerator.

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